The present invention is directed to controlling and monitoring circuits for electrical seat heating means, especially of automotive vehicles, comprising a control circuit including an electric heating element, a measuring section energized in response to the resistance of the electric heating element, and switching means controlled by the output from the measuring section for turning the electric heating element off.
With seat heating means which for example in the vicinity of the seat cover comprise electric heating elements formed by heating wires, which are provided on a substrate for example in a meander-like pattern, a zig-zag pattern or the like, short-circuits or interruptions of the heating wires may occur under load during use. Short-circuits which bridge certain sections of the length of the heating wire in the electrical resistance-type heating element of the seat heating means will result in increased current consumption and in the formation of hot spots on the heated seat or back rest; these faults cannot be detected by control circuits for controlling the power supply to the seat heating means when overheating occurs in areas of the electrical resistance-type heating element which are not thermally coupled to a temperature sensor of the control circuit.
To overcome these difficulties, the heating current through the electrical resistance-type heating element has already been monitored by a suitable measuring section which provides a signal causing the electrical heating element to be turned off when the heating current rises above a predetermined level, for example due to internal short-circuits within the seat heating means.
But such a monitoring circuit exhibits the drawback that the measuring section must be designed for the magnitude of the current flowing through the electrical resistance-type heating element in accordance with the respective heating capacity, or that in case of the electrical heating element being supplied with an a.c. current, relatively complex and expensive current transformers must be used for deriving a smaller measured current representative of the heating current.
Therefore the invention is intended to provide a controlling and monitoring circuit for electrical seat heating means of the specified kind in such a way that any faults of the electrical resistance-type heating element, especially internal short-circuits resulting in local overheating, can be detected reliably and extremely fast in a simple monitoring circuit which operates with very small measuring currents and causes the seat heating means to be turned off. Such a controlling and monitoring circuit is intended to respond to a multiplicity of conceivable faults of the electrical seat heating means and to sense such faults within a wide range of operating conditions.